1. Field of the Invention
The present invention relates to a method for manufacturing a magentic bubble memory.
2. Description of the Prior Art
As is well known, a prior art magnetic bubble memory usually uses permalloy propagation tracks.
As shown in FIG. 1, propagation tracks (permalloy pattern) 1 made of permalloy (iron-nickel alloy) are formed on a magnetic film (not shown) capable of holding magnetic bubbles such as (YSm Lu Ca).sub.3 (FeGe) .sub.5 O.sub.12 with an insulative film being interleaved therebetween, and a magnetic bubble 2 formed in the magnetic garnet film is propagated along the propagation track 1.
A transfer gate, a swap gate and a replicator are constructed by a conductor pattern 5 shown in FIG. 2 which is formed between the magnetic garnet film 3 and the permalloy pattern 1 with insulative films 4 and 6 being interleaved therebetween. By supplying a controlling pulse current to the conductor pattern 5, various functions are attained.
In order for the permalloy device to operate normally, a positional relation between the permalloy pattern 1 and the conductor pattern 5 is very important. Accordingly, it is essential to enhance mask alignment accuracy when those patterns are formed by photolithography.
On the other hand, as memory density and integration density of the magnetic bubble memory increase, a pattern width and a gap (pattern-to-pattern spacing) of the permalloy pattern have become very small. In order to further increase the integration density, it is necessary to reduce the pattern width and the gap to less than 1 micron. It is, however, difficult to form such a fine permalloy pattern by the prior art photolithography and hence it is very difficult to significantly increase the integration density of the permalloy device.
In order to overcome the above-noted difficulty, an ion implanted device (contiguous disk device) has been proposed which is fabricated by implanting H.sub.2.sup.+ or Ne.sup.+ ions to the magnetic film through a contiguous mask (moniliform mask) formed on the magnetic film to form an ion implanted region 7 and a moniliform non-ion implanted region 8 of contiguous disk shape on the magnetic layer 3 as shown in FIG. 3.
In such a device, the moniliform non-ion implanted region 8 acts as the bubble propagation track and the bubbles are propagated along an outer edge of the contiguous disk patterns 8.
In the ion implanted device, the propagation track can be more readily formed than that of the prior art permalloy device because no gap is included in the contiguous disk patterns 8. Accordingly, it is very advantageous to increase the integration density.
However, the ion implanted device has a disadvantage in that stability of operation of the replicator, the transfer gate and the swap gate is low. Accordingly, a hybrid device in which a propagation track of a minor loop is formed by the ion implantation and a propagation track of a major loop is formed by the permalloy film has been proposed.
As shown in FIG. 4, the hybrid device has the ion implanted region 7, the conductor pattern 5 and the permalloy pattern 1. Accordingly, three steps of mask alignment are needed to form the hybrid device. The prior art permalloy device which has no ion implanted region 7 can be formed through two steps of mask alignment. Thus, the reduction of the manufacturing accuracy and the increase in the number of the manufacturing steps due to the increase of the mask alignment steps are obstacles to be resolved.